1. Field of the Invention
The present invention relates to the field of voltage converters and in particular to a charge pump voltage converter.
2. Description of the Related Art
Many electrical devices require power supplied at a stable voltage different than that provided by a primary power source. In many applications, the primary power source is a battery. Often the electronic devices require voltages that are between 1 and 2 times the voltage provided by the battery. An additional requirement is that the voltage provided be relatively stable. A low voltage can result in the powered devices failing to operate at all or at a reduced performance level. Steady overvoltage can reduce the life of the devices or permanently damage the devices. Spikes or transients in voltage can also disrupt device operation and cause damage.
One difficulty with batteries is that many batteries do not provide a stable output voltage. The output voltages of many batteries decrease as the batteries are used and as the batteries age. The voltages can also vary depending on how heavily the batteries are loaded. Certain batteries also vary in output voltage with variations in temperature. Even under conditions where the battery voltage is not varying, the battery may provide power at a different voltage than that required by user devices.
Charge pumps are known circuits that effectively transfer electrical charge back and forth between storage components to generate an output voltage different from an input voltage. Charge pumps with a “buck” feature are effectively voltage limiters. If the input voltage exceeds a threshold value, the charge pump “bucks” the overvoltage away from the load. However, in a charge pump circuit, the charge is not simply shunted to ground or another load as in, for example, zener diode circuits. A charge pump temporarily stores the charge redirected from the load, typically in a capacitive element. This charge stored in the capacitor is then typically delivered to the load at a later time. Zener diodes are effective at clamping voltages above a certain threshold; however, by simply shunting the current away from the load, the current is typically not available for use. This results in wasted power. It will be appreciated that wasting power in a device with limited battery capacity is preferably avoided.
In a “boost” operation, a charge pump accumulates charge to be able to provide a greater voltage to the load than is provided by the input voltage. A charge pump in boost operation typically sequentially charges at least one capacitor connected in parallel with the power source and then selectively interconnects the capacitor(s) in series with power source to increase the available voltage for delivery to the load. Typical charge pump circuits double or triple the input voltage minus some switching and other loses. Again, it will be appreciated that minimizing loses from a limited power source such as a battery is desirable.
Charge pump circuits are often used in consumer electronics, such as PDAs, cell phones, and the like. Thus, it will be appreciated that simplicity and low cost are highly desirably. With potential markets in the millions of units, a reduction in cost of only a few cents can add up to significant savings and increased profits for the manufacturers and sellers. An additional design goal is to reduce size and weight of the devices. Reduced size and weight increases the convenience of an appliance to the consumer and increases the marketability of the appliance. Many known charge pump designs employ multiple operating modes that increase circuit complexity and cost of the charge pumps. Multiple operating modes also generally lead to voltage transients upon switching between the multiple modes, which again can damage powered devices.
An additional problem with the charge pump circuits is that the conversion efficiency declines very quickly when the output voltage is less than twice the input voltage. The general rule for any charge pump is that input current will always be twice the output current when the circuit is in equilibrium. Current is drawn from the battery on every clock cycle; however, it is only supplied to the load every other clock cycle. Thus, the instantaneous current is the same in the input and output sides, but the time average current for the input is twice the output. Since efficiency is the ratio of power out divided by power in, and input current is always twice output current, the maximum theoretical efficiency can be calculated as follows:Eff=Pout/Pin Eff=(Iout×Vout)/(Iin×Vin) Eff=(Iout×Vout)/(2×Iout×Vin) Eff=(Vout/(2×Vin))×(100%) Therefore:Eff=100% max when Vout=2Vin Eff=50% max when Vout=Vin Eff=25% max when Vout=½Vin 
In practical circuits, it is reasonable to expect 10% losses in the converter because of resistive losses in switching components and junction drops. The actual predictions made by multiplying the theoretical predictions above by 90% are 90%, 45% and 22%, respectively.
One reason for the low efficiency numbers when Vout is <2 Vin is that the charge current for the transfer capacitor Cx flows from the battery directly to ground without imparting its full energy on Cx, (i.e., Cx is not charged to the battery's full potential). Thus, the voltage difference between the available battery potential and the voltage needed to charge Cx to obtain the desired output voltage is available, but is not utilized.
Many real world battery-powered applications that include multiple loads with different power requirements and meet the criteria for using a secondary load output from the charge pump. For example, cellular telephones and handheld computers (PDAs) are typically powered by single cell Lithium-ion or triple cell NiCad batteries having terminal voltages that range from 5.6 volts at full charge to 3.0 volts at cutoff. A common requirement in these products is to provide one regulated output voltage in the 5-volt to 3.3-volt range and to provide a second regulated output voltage in the 2.5-volt 1.5-volt range.
A very specific application in the cellular phone product area is driving a combination of white and green LEDs that light a color display and a keypad. White LEDs are needed to provide good color from a liquid crystal thin film transistor (TFT) display. These white LEDs typically have forward voltage drops of 3.6V and draw approximately 20 mA each. Therefore, the white LEDs require a buck/boost voltage converter to operate from the normal 5.6-volt to 3.0-volt battery potential. An additional requirement for the white LEDs is that all the LEDs generate approximately the same light intensity in order to achieve uniform lighting in the display.
The cell phone also uses a lighted key pad, but this can use lower cost and lower voltage green LEDs. Uniformity of lighting and color trueness of the keypad is less of a concern than with the display. Green LEDs operate at 2.0 volts at 10 mA drive levels, thus using less power per LED (20 mW) than white LEDs (72 mW). However, because the electrical requirements of green and white LEDs differ, two separate circuits are typically required to enable to advantages of using green and white LEDs.
In order to maintain a constant and consistent light output, multiple LEDs require a constant current rather than constant voltage. One method of driving LEDs is to use a constant voltage source with current limiting ballast resistors in series with the LEDs to sense and/or control forward current. Multiple LEDs can be driven in parallel or in series. If in series, only one series resistor is required for the LEDs in that branch, however the supply voltage must be high enough to support the sum of the forward voltages of the LEDs. Unfortunately, the voltage required for two or more LEDs is higher than readily achievable with a switched capacitor charge pump fed by a typical battery.
When multiple LEDs are driven in parallel, the supply voltage only needs to be in the 4V range, which is easily achievable with a charge pump operating from a 3-volt battery. In the parallel case, each LED has its own series resistor to control and balance its current. However, this approach has two weaknesses:                1. Current matching among the LEDs is needed to insure equal light output. Individual LEDs will have different forward voltage drops at equal drive currents. Because of this, the value of a series ballast resistor must be fairly high to control current sharing without undertaking the significant time and expense of testing and selecting LEDs for minimal variations in forward voltage. Typically, dropping at least one-fourth of the forward voltage of the LEDs would be needed to maintain less than 10% current variation among the multiple LEDs. With a primary supply voltage of 3.0 volts providing power to a charge pump powering white LEDs with 3.6-volt forward voltage drops, 0.9 volts would typically be needed across the series resistor to achieve less than 10% current variation. This would result in a 20% loss because one-fifth of the total supply voltage is used in the ballast resistor and energy is lost to resistive heating rather being used for the desired light production.        2. The product would be burdened with the extra space and cost of the series resistors.        
Thus, from the foregoing it will be appreciated that there is a need for an efficient charge pump that provides both buck and boost operations and that provides an output voltage that is regulated to provide a stable output voltage even in the presence of variations in an input voltage. A need also exists for a regulating charge pump of simple design that avoids the cost and complexity of multiple operating modes. A need also exists for a charge pump that avoids switching between multiple operation modes and that minimizes switching transients. Furthermore, a need exists for a buck/boost capable charge pump that can provide regulated outputs at different voltage levels with a single circuit. Advantageously, such a multiple output regulating charge pump operates with improved efficiency. Moreover, a need exists for a single circuit that provides multiple voltage regulated outputs and that also regulates the current in multiple branches of at least one of the outputs so as to facilitate powering LEDs in a highly efficient and balanced manner.